A flash memory device is a highly integrated information storage device in which write and erase operations are accomplished on board. A flash memory cell includes at least one field effect transistor (FET) having a selection gate, a floating gate, a source, and a drain. Threshold voltage variation results in charge fluctuation on the floating gate. The charge fluctuation is data that is stored in the flash memory cell.
Flash memory devices are classified into NAND and NOR devices. The NAND flash memory device is used as a mass data storage device, while the NOR flash memory device is an information storage device used in high speed data processing applications.
The NOR flash memory cell has two states, i.e., a programmed state and an erased state. When the NOR flash memory cell is programmed, residual electrons are trapped on a floating gate and a threshold voltage rises. Thus, drain-source current does not flow to a selected flash memory cell. The programmed state of the flash memory cell is typically a logic “0”. When the flash memory cell is erased, there are a small number of residual electrons on the floating gate or lots of source-drain current flows to the flash memory cell. The erased state of the flash memory cell is typically a logic “1”.
A conventional flash memory device is partially illustrated in FIG. 1. Referring to FIG. 1, a flash memory device 100 includes a memory cell array block 110, a row decoder 120, wordline level selecting units 130, a driver unit 140, and a page buffer 150. The memory cell array block 110 has i strings 200, 202, 204, and 206. Flash memory cells are serially coupled to the strings 200, 202, 204, and 206. Specifically, flash memory cells, e.g., 112, 113, and 114, are serially coupled to first to sixteenth wordlines WLj (where j=1-15). A drain of the flash memory cell 112 coupled to the first wordline WL0 is coupled to a first selection transistor 111 whose gate is coupled to a string selection line SSL. A source of the flash memory cell 114 coupled to the sixteenth wordline WL15 is coupled to a second selection transistor 115 whose gate is coupled to a ground selection line GSL. A drain of the first selection transistor 112 is coupled to a first bitline BL0, and a source of the second selection transistor 115 is coupled to a common source line CSL.
The strings 200, 202, 204, and 206 are coupled to the string selection line SSL, the first to sixteenth wordlines WLj (j=0, 1, . . . , 15), and the ground selection line GSL. Each of the strings 200, 202, 204, and 206 is connected to the page buffer 150 through bitlines BL0, BL1, . . . , BLi-1, and BLi to constitute a page unit. Although FIG. 1 shows only one page in the memory cell array block 110, the memory cell array block 110 might include a plurality of page units. The page units are addressed by the row decoder 120. Transistors in the driver unit 140 are turned on to select a page unit responsive to an addressing signal from the row decoder 120.
The wordline level selecting units 130 selects one of a program voltage VPGM, a read voltage VREAD, a program verify voltage VRDV, and a pass voltage VPASS in accordance with the corresponding mode. The program voltage VPGM is applied during a programming operation of selected flash memory cells. The read voltage VREAD is applied to during a reading operation of the selected flash memory cells. The program verify voltage VRDV is applied while verifying programming of the selected flash memory cells. The pass voltage VPASS is applied to unselected flash memory cells, so that cell current can flow or pass. The driver unit 140 applies a corresponding mode voltage to the wordlines WLj (j=0, 1, . . . , 15) in one selected page unit responsive to the addressing signal of the row decoder 120. The page buffer 150 senses the cell current flowing through the bitlines BL0, BL1, . . . , BLi-1, and BLi to verify data of selected flash memory cells.
The above-described flash memory device 100 iteratively repeats a program verify operation (i.e., “program unit loop”) in which the flash memory device 100 programs the flash memory cells and then verifies whether cells were programmed. If a flash memory cell is normally programmed, a program operation stops. If not, the program operation is performed one more time and the program verify operation is iteratively repeated. For example, assuming that there are 4K flash memory cells coupled to a wordline WL, predetermined cycles are needed for programming the flash memory cells and verifying that the flash memory cells are programmed. If the cycles are iteratively repeated to verify that the flash memory cells coupled to the wordline WL are completely programmed, a program operation and a program verify operation of another wordline WL are repeated.
A program operation and a program verify operation of a selected wordline WL are iteratively repeated until no under-programmed flash memory cells exist. Once that occurs, the program operation ends. As shown in FIG. 2, the program voltage VPGM rises and the program verify read voltage VRDV is constantly maintained while one operation unit loop is iteratively conducted. During the program unit loop, cell current is intensifies by the under-programmed flash memory cells to raise the level of the common source line CSL. If a constant program verify voltage VRDV is applied during a subsequent program unit loop, the level of the common source line CSL is raised to weaken the cell current. The result is that cells might be determined programmed when they are not.
The threshold voltage Vt of a flash memory cell has distribution based on the program verify method of FIG. 2, which will be described with reference to FIG. 3.
Referring to FIG. 3, in an initial state where an erase operation for all flash memory cells is conducted, the threshold voltage Vt of the flash memory cells has a negative voltage level. The program unit loops are sequentially conducted. In the last program unit loop, flash memory cells are programmed by a high program voltage VPGM, so that the threshold voltage has a high and non-uniform distribution. The non-uniform distribution of the flash memory cells having the high threshold voltage Vt causes flash memory cells of a page unit to be incompletely erased in an erase operation (over-program). Since it is unclear whether the flash memory cells having a low threshold voltage that is close to the program verify voltage VRDV are programmed, they remain under-programmed.